Hyperfine and Superhyperfine Tensors as Probes of the Local Environment of Deep-Level Defect Centers
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HYPERFINE AND SUPERHYPERFINE TENSORS AS PROBES OF THE LOCAL ENVIRONMENT OF DEEP-LEVEL DEFECT CENTERS MICHAEL COOK AND C.T. WHITE *Department of Chemical Engineering,
University of Massachusetts,
Amherst,
MA 01003, and Geo-Centers Inc., Ft. Washington, MD 20744 **Theoretical Chemistry Section, Code 6119, Naval Research Laboratory, Washington,
D.C.
20375
INTRODUCTION Point defects occur in every solid material. No crystalline lattice is perfect, and no amorphous network has only unbroken sequences of bonds. Every material contains a greater or smaller number of vacancies, interstitials, substitutional atoms, and broken bonds. Many of these have only minor effects on the behavior of the material, but in a surprisingly large number of cases, point defects can have significant and even decisive effects on material performance. This can be true even when the defects are present in very small concentrations. In electronic device materials, for example, broken-bond defect centers are particularly serious; they generate unsatisfied hybrid orbitals that can act as electron or hole traps, hindering carrier transport and degrading 1 device performance. In energetic materials, defect centers can have even 2 more dramatic effects. The scission of bonds within or around nitro- and nitramine groups creates radical species that are typically more reactive than the bulk solid, increasing the sensitivity of the material to initiation.
Technologically important defects have been investigated by a broad range of experimental methods, particularly in device materials where many electrical techniques are available. However, the most consistently useful methods for characterizing defect structure have been magnetic resonance techniques, particularly electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR). Deep-level centers are often paramagnetic in one or more of their commonly occurring charge states. ESR and ENDOR probe the interaction of the magnetic moment of these unpaired electron spins with an applied magnetic field, without background interference from the diamagnetic bulk of the material. The observed spectra can give very precise information about the distribution of electron spin density in the neighborhood of the defect center, and consequently about its detailed structure. 3 These techniques have been used in small-molecule chemistry for many years, and have been a staple of solid-state defect analysis since the classic papers of Watkins and Corbett.4'5 Much of our knowledge of the nature and structure of defect centers in materials has been derived from ESR and ENDOR data. Most of the theoretical interpretation of ESR spectra has remained at a simple interpretative level, in which the g-tensor and the hyperfine interactions are used only to derive estimates for the hybridization and degree of delocalization of the defect orbital. In this article we go a step beyond that level of analysis, and use the structure of a simple tightbinding model to derive an approximate relation between the superhyperf
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